The present invention relates to locating and securing a specific position on a living organism at which an external device may be placed to facilitate optimal signal coupling between an external device and an implanted device, for purposes of data or power transfer in either direction. In particular, the device of the invention may be used with implantable stimulators, such as an implantable intracochlear prosthesis for electrically stimulating the auditory nerve of a profoundly deaf person, or a neuralmuscular stimulator for functional electrical stimulation and sensing.
Devices are known in the art that provide the sensation of hearing for the profoundly deaf by electrically stimulating the auditory nerve neuronal endings along the basilar membrane within the cochlea. Such devices commonly comprise a supercutaneous unit, which detects sound and encodes and transmits signals representative of the sound, and a subcutaneous unit for receiving these transmissions and directly stimulating the auditory nerves accordingly. Such a device is contained, e.g., in U.S. Pat. No. 5,095,904.
Implantable neuralmuscular stimulators are also becoming increasingly sophisticated, with some models receiving power via transcutaneous coupling, and with most models transmitting back to an external device a great deal of information about the stimulator and the signals it senses in a patient's body.
Alignment of the supercutaneous unit with the subcutaneous unit is critical in these devices for effective electromagnetic coupling. In the past, the external unit has been taped to the skin. Taping, however, can be unsightly, and furthermore uncomfortable to the wearer of the device. Additionally, should the tape come off, it may be difficult to relocate the proper position for effective coupling.
Another method known in the art to align units used for the hearing impaired is to build an external device into eyeglass frames. Unfortunately, the frames are prone to slipping and misalignment, causing a decrease in or loss of the signal coupled transcutaneously.
Yet another device known in the art includes a permanent magnet in each of the implanted and external devices, as described in U.S. Pat. No. 4,352,960, issued to Dormer et al., now U.S. Pat. No. Re. 32,947. Rare-earth magnets in both the implanted and external units magnetically secure the external unit in the proper position for optimal coupling. A drawback of such design is that the magnets can significantly reduce the internal space available within the implanted unit, and increase the weight of both the internal and external units. Furthermore, for small internal units, which require the use of smaller magnets, the magnets provide only a weak magnetic coupling, which is insufficient to hold the external unit in place and/or is unreliable. On the other hand, the magnets can cause excessive compressive pressure to be applied to tissue of the living organism. Such excessive pressure causes a reduction in blood flow through the tissue and, as a result, the deterioration of such tissue. Magnets can also saturate the cores in adjacent transformers or coils causing them to function improperly.
In view of the above, it is clear that what is needed is a means of conveniently attaching an external unit to a proper location on the skin of a living organism for optimal coupling to an implanted unit, such as a cochlear stimulator, which attachment means is reliable, low-cost, and does not engender possibly serious medical side-effects.